Levelling system for bowling lane sanding machine



Oct. 19, 1965 D. c. sAMs ETAL LEVELLING SYSTEM FOR BOWLING LANE SANDINGMACHINE Filed July 5, 1962 5 Sheets-Sheet 1 INVENTOR DAVID C. SAMSRICHARD D. WALSH 5 Sheets-Sheet 2 INVENTOR DAVID C. SAMS BY RICHARD D.WALSH Oct. 19, 1965 D. c. SAMS ETAL LEVELLING SYSTEM FOR BOWLING LANESANDING MACHINE Filed July 5, 1962 Flegz Oct. 19, 1965 c. s s ETAL3,212,218

LEVELLING SYSTEM FOR BOWLING LANE SANDING MACHINE Filed July 5, 1962 5Sheets-Sheet 3 INVENTOR DAVID C. SAMS BY RICHARD D. WALSH Oct. 19, 1965D. c. sAMs ETAL LEVELLING SYSTEM FOR BOWLING LANE SANDING MACHINE FiledJuly 5, 1962 5 Sheets-Sheet 4 INVENTOR DAVID C. SAMS RICHARD D. WALSH Q.21%

Oct. 19, 1965 D. c. SAMS ETAL 3,212,218

LEVELLING SYSTEM FOR BOWLING LANE SANDING MACHINE Filed July 5, 1962FIG. 6

5 Sheets-Sheet 5 FIG. 7

CRla

CR2b TO LEFT TO RIGHT MOTION MOTlON INVENTOR DAVID C. SAMS B RICHARD D.WALSH United States Patent O LEVELLHNG SYSTEM FOR BOWLING LANE SANDINGMACHINE David C. Sams, Plymouth, and Richard D. Walsh, Shelby,

Ohio, assignors to American Machine & Foundry Company, a corporation ofNew Jersey Filed July 5, 1962, Ser. No. 207,501 6 Claims. (Cl. 51--174)This invention relates to sanding machines, and more particularly to theautomatic levelling of machines of the type used where perfectlyhorizontal sanding is a critical requirement.

The invention will be described in its application to a machine forsanding bowling lanes, because it is in such sanding that uniformity ina horizontal plane perhaps reaches its greatest imporatnce. However, theinvention is also applicable to other surface finishing operations of asimilar nature. In the sport of bowling, the surface condition of a laneis a highly significant factor entering into movement of a ball towardthe target area. In order to be approved for official play by thevarious bowling associations, one requirement that is imposed is thateach lane must be level from side to side within close tolerances.Bowling lanes periodically must be given a complete refinishing and inthe sanding operation it obviously is desirable to maintain a levellingcontrol over the machine in order that trueness of the lanes will bemaintained. Otherwise, the errors produced by uneven sanding can becomecumulative until finally a lane deviates considerably from a horizontalplane.

One approach to levelling control has been to provide means which sensethe degree of trueness of the lane and to regulate the cutting actionaccordingly. As a sanding machine proceeds along the lane it isnecessary to detect even slight deviations from level and responsivelyto effect the necessary corrective adjustments as quickly as pos sible.Highly satisfactory sanding of a lane can be accomplished only by theearly detection of very small deviations and, ideally, by the continuousinitiation of correspondingly small corrective adjustments. To this endthe level sensing means must, among other requirements, be highlysensitive and accurate.

Accordingly, the main object of the invention is to provide improvedsensing means, in the nature of a clinometer, of the type described. Inaccordance with the invention the improved sensing means as a unit ishighly sensitive and accurate and in this and other respects is highlysatisfactory for the control of a machine for sanding a bowling lane orother floor surface. Briefly, in accordance with the invention a bubbletype liquid level system is employed in conjunction with a novelelectronic control circuit. Specially, two levels in the form of vials,each containing an electrically conductive liquid, incorporateelectrical contacts or electrodes which produce different effects inresponse to conditions of levelling. One of the vials reacts upon a veryslight deviation away from a horizontal reference in one direction andthe other vial reacts upon a slight deviation away from a horizontalreference in the other direction. In the preferred embodiment of theinvention, each vial is coupled to its own independently operatingcircuit, each circuit being operative to initiate a given correctivelevelling adjustment. When either of these circuits reacts to itsassociated levelling vial unit it produces an output of pulses whichdrive one of two electromagnetic devices. Each of these devices isoperable to effect a levelling adjustment in a given direction. Certainadvantages inhere in this pulse type of operation. Further details ofthe invention, however, will be more readily understood from thedetailed description provided hereafter.

Other objects and advantages also will appear from the followingdescription, read in conjunction with the accompanying drawings, inwhich:

FIGURE 1 is a side elevation of an entire bowling lane sanding machinein which the present invention is embodied;

FIGURE 2 is a partial plan view of the sanding roller assembly;

FIGURE 3 is a side elevation of the structure taken as indicated bylines 3-3 in FIGURE 2;

FIGURE 4 is a sectional side elevation taken as indicated by lines 4-4in FIGURE 2;

FIGURE 5 is a sectional end elevation taken as indicated by lines 55 inFIGURE 4;

FIGURE 6 is a schematic illustration of the bubble type liquid levelsand their associated electrodes; and

FIGURE 7 is a wiring diagram of the electrical control circuit inaccordance with the invention.

FIGURES 1 and 2 present top and side views of an entire sanding machine,although as previously indicated this invention concerns principally theautomatic levelling of certain machine partsthe sanding pressure drum inthe instant caseto maintain trueness of a bowling lane or other floorsurface. in the particular machine illustrated three drums are involved,namely, a driving drum 2, a cutting or pressure drum 4 and an idler drum6. A suitable endless abrasive belt 8 of a width sufficient to cover thefull width of a bowling lane (typically 4' to 42 inches) is trainedabout these drums and is rotated in a clockwise direction (in FIGURE 1)by means referred to hereafter. It is to be noted that this sandingmachine is of the type wherein the sanding belt travels in align mentwith the length of the bowling lane as distinguished from travel at aright angle to the lane.

The machine frame consists essentially of two parts. Forwardly, themachine frame is a weldment comprising spaced side members it] and 12spanned by cross members 14 and 16. At the left-hand side of the machinethe member 12 is detachably connected to the front cross member 18 by apivotally connected plate 20 which is secured to the member 12 byspecial intermeshing elements (not shown) and a thumb screw 22. Therearward machine frame is a weldment comprising spaced side members 24spanned by cross members 34, 36 and 38. The left-hand side frame members12 and 28 are detachably connected by a plate 36 and thumb screws 32 Thepurpose of these detachable connections is, that by removing rearwardplate 30 and swinging away forward plate 2%) the side frame is broken atthese locations to permit sanding belts to he slipped on and off thedrums 2, 4 and 6. In the course of this operation the forward portion ofthe sanding machine is raised above the lane or other floor surface (bymeans described hereafter) and to prevent the dropping of the thenbroken left-hand side there is incorporated a feature which is describedin the copending application of John Zuercher, Serial No. 207,503 filedJuly 5, 1962, now Patent No. 3,123,944 and entitled Frame Structure forSanding Machine. Briefly, this feature involves a torque bar principlewhereby the right-hand side of the frame acts to maintain suspension ofthe broken away left-hand side of the frame. As disclosed in saidcopending application, to which reference may be made, in practice thisis effected by coaction between an arm 26 and the forward cross member36 of the rearward machine frame.

The forward machine frame is cantilevered outwardly from the rearwardmachine frame, the raising and lowering of the forward machine framerelative to the bowling lane or other floor surface being effective tovary the cutting pressure. The rearward machine frame is supportedpartly by heavy wheels 40 which are keyed to a transverse axle 42 drivenby an electric motor 44 through a gear reduction unit 46, and which alsoserve to drive the sanding machine along the bowling lane or other floorsurface. The remaining support for the sanding machine is provided bycaster wheels 48 pivotally carried by a lead screw 50 which is axiallyadjustable by its rotation within a stationary nut assembly 52. A frame56 extending upwardly provides handle bars and a support for the machineoperators controls. The lead screw 50 is rotated to raise and lower therear of the machine relative to the caster wheels 48, thereby to raiseand lower the cantilevered forward pressure drum 4 by rotation about theaxis of axle 42. Lead screw 50 is rotated by means of a universal driveshaft 54 which is manually operated by a handwheel 58. It will beunderstood that by the operation of handwheel 58 the pressure drum 4 maybe raised completely above the floor surface (e.g. for the purpose ofchanging sanding belt 3) or on the other hand may be lowered to applylight or heavy pressure to the floor surface being sanded.

In the course of movement of the sanding machine along a bowling lanethe machine is maintained in paral lel, centered relation to the lane byrollers 59 which ride along the division boards or other structureseparating adjacent bowling lanes.

The finish which is removed from the floor surface in the form of dustis delivered to a closed dust container 60 by centrifugal blowers 62which are connected to a collector duct 66 by ducts 68. A fireextinguishing system is provided which is the subject matter of thecopending application of Arthur B. Viescas, Serial No. 207,504, filedJuly 5, 1962, now Patent No. 3,146,558 and entitled Fire ExtinguishingSystem for Sanding Machine. Briefly, this system comprises a tank 72containing an approved fire extinguishing agent (such as carbon dioxide)which is connected by a line 74 to the intake duct 76 of the dustcontainer 60. A valve 75 is operated to admit the fire extinguishingagent to the dust container 60 in response to a signal emitted by means(not shown) responsive to the temperature condition within the dustcontainer.

The driving drum 2 is rotatably supported in bearings 78 and 8d at theside frame members and 12, respectively. An electric motor 82 is mountedon a subframe assembly comprising cross members 83 and side members 24.Motor 82 drives a disk type clutch 86 through a belt drive 84, theclutch 86 being engageable to drive an output shaft 88 which in turn isconnected to the drum 2 by a belt drive 90. The engagement anddisengagement of clutch 86 is effected through operation of a lever 92by a control lever 94 at the machine operators position. Motor 82 alsodrives the previously mentioned centrifugal blowers through a belt drive96.

Idler drum 6 is rotatably mounted at its left end by a bearing 102, andat its right end by an assembly generally designated 104. Assembly 104is a pneumatically operating unit, the details of which need not bedescribed for purposes of the present invention. Briefly, however, theassembly 164 provides a bearing 105 which is cyclically shifted back andforth on a vertical line by the very gradual reciprocable operation of apneumatic motor 107. The resultant continuous shifting of the axis ofdrum 6 causes a slight lateral movement of the belt 8 back and forth onthe drums 2, 4 and 6. The purpose of such belt shifting is to produce amore uniform sanding of the bowling lane or other floor surface. Thepneumatic motor 107 is responsive, in its reversals of movement, to apneumatically operating nozzle and baffle unit 109 which senses thelateral shifting of the sanding belt 8 between predetermined limits.

At each side of the sanding machine a toggle structure 106 is providedto be used in moving the idler drum 6 backwardly, thereby to release thetension on the sanding belt 8 and aid in its removal from the drums.

At the front of the sanding machine a vertically adjustable roller 108is provided. This roller is so adjusted as to engage the floor surfaceand thereby limit the bite of the cut only under unusual conditions.

The pressure drum 4 is rotatably supported by bearings 110 mounted in arectangular levelling frame comprising side plates 112 and 114 and crossplates 116 and 118. The levelling frame is mounted in the nature of atrunnion having the mutual axis of its pivots extending lengthwisethrough the machine and intermediate the left and righthand sidesthereof. These pivots are indicated at 120 and 122, the fixed portionsof the bearings being mounted on the frame cross members 16 and 14,respectively. It will be evident from the description that by virtue ofthis mounting of the levelling frame the pressure drum 4 supportedtherein can be maintained in exactly horizontal disposition despitedeviations of the main machine frame from the horizontal because ofunevenness of the bowling lane or other fioor surface being sanded. Thisbasic mechanical arrangement for the mounting of the pressure drum 4,including the hereafter described cam assembly for effecting theindicated necessary angular adjustments of the levelling frame relativeto the main machine frame, is the subject matter of the copendingapplication of Harold W. Martin, Serial No. 207,502, filed July 15, 1962and entitled Sanding Machine.

Suspended by legs 123 depending from side frame plates 112 and 114 is aplaten 125 which has a concave upper configuration conforming to thedrum 4 and a flat bottom surface. Between the belt 8 and this bottomsurface there is provided a graphiteimpregnated length of canvas 127.Platen 125 is used when making the finer finishing passes across thefloor surface, and when the platen is so used the drum 4 does not rotatebut merely applies force thereto. When the preliminary rough cuttingpasses are being made the platen may be removed and the drum 4 will thenrotate. When platen 125 is not used the weight transmitted through drum4 is applied to the floor surface over a much smaller area than when theplaten is used, and consequently the cutting pressure will beconsiderably greater in the former case.

The rearward levelling frame cross plate 118 has at its left a cam track124 which is inclined downwardly and centrally toward the right (inFIGURE 3) and a cam track 126 which is inclined downwardly and centrallytoward the left. A lead screw 123 is freely journalled for rotation in agear box 1311 mounted on the frame member 12. A traveling nut 134 onlead screw 128 is joined with a carriage 136 having an upper roller 140,which rides flatly against a stationary member 142 forming an extensionof the main machine framework, and having a lower roller 138 whichstraddles the cam track 124. Toward the right-hand side of the machine asecond carriage 146 similarly has an upper roller 150 which ridesagainst the member 142 and a lower roller 148 which straddles the camtrack 126. Carriage 146 is connected to carriage 136 by a connectinglink 144 and thereby a given movement of carriage 136 will result in anidentical movement of the carriage 146.

A right angle bevel gearing unit 152 is constantly driven by anextension of the shaft of drum 2 and has a constantly rotating outputshaft 154. A pair of normally free-wheeling electromagnetic clutches 156and 158 are concentric with shaft 154. Within the gear box 130 there isa differential gearing generally indicated 160, through which the leadscrew 128 may be rotated in either direction. Upon energization ofclutch 158 the shaft 154 will drive the differential through belt drive162, and upon energization of clutch 156 the shaft 154 will drivedifferential 160 through belt drive 164. If magnetic clutch 158 isenergized the lead screw 128 will rotate in a direction to movecarriages 136 and 146 toward the right and if clutch 156 is energizedthe lead screw 128 will rotate in a direction to move carriages 136 and146 toward the left in FIG- URE 3. When the carriages 136 and 146 movetoward the right, for example, the roller 148 will force the levellingframe plate 118 in a clockwise direction and thereby tend to lower theright-hand side While tending to correspondingly raise the left-handside of drum 4. If neither of the clutches 156 and 158 is energized, thedrum 4 will maintain its position relative to the main machine frame. Itwill be understood that these angular adjustments of the drum 4 aboutthe axis of pivots 120 and 122 Will be effected as necessary to maintainthe axis of the drum in exactly horizontal disposition despiteinclination of the main machine frame due to unevenness of the bowlinglane or other floor surface. Thereby the floor surface will be sandedevenly, in a truly horizontal plane.

Mounted on the levelling frame side members 112 and 114, and subject todeviations thereof from horizontal position, is a beam 166. Beam 166 hasdepending legs 168 which are detachably connected to levelling frameextensions 170 by means of latches 172. It should be understood,therefore, that with respect to the horizontal the beam 166 will assumethe same angular relation as the axis of pressure drum 4. Afiixed to thebeam 166 is a pair of bubble type liquid levels or vials, indicated 174and 176. These vials are identical, and each comprises a glass containerhaving a slight upward curvature as in the case of any bubble typelevel. The vials 174 and 176 contain an electrically conductive liquid,preferably methyl alcohol, and electrodes leading from within the vialsto an electrical control circuit control the operation of this circuitin response to level or non-level conditions of beam 166. The vial 176will be referred to herein as the cut right vial because it operates toeffect a corrective levelling adjustment when the right side of thebowling lane or other floor surface is sensed as being higher than theleft side. The vial 174 will be referred to herein as the cut left vialbecause it operates in the opposite manner. As illustratedschematically, the vial 176 has embedded therein a tap or electrode 178which is always immersed in the conductive liquid L and therefore isunaffected by movements of the bubble B. Protruding through the top ofthe vial 176, however, is a tap or electrode 180 which is insulated fromthe liquid L by the air gap in bubble B when the vial 176 is inperfectly level position, but which will be immersed in the liquid L iftilting causes the bubble B to move toward the right. In other words, ifa voltage is applied across the electrodes 178 and 180 a signal currentwill not be conducted through the conductive liquid L and the electrodeswhen the vial 176 is level, but will be conducted when the bubble Bshifts to the right by deviation of the vial 176 from the horizontal ina counterclockwise direction. Similarly, the vial 174 has a protrudingtap or electrode 172 which is constantly immersed, and an upper tap orelectrode 184 which is insulated from the conductive liquid L when thebubble B occupies the central, or level position. In this case, however,if the bubble B moves to the left due to a deviation of the vial 174from the horizontal in a clockwise direction, a signal current can thenbe conducted through the electrodes 182 and 184 via the conductiveliquid L.

Summarizing what has just been stated, with applied voltages across therespective pairs of electrodes 178, 180 and 182, 184, both pairs ofelectrodes will be nonconducting when the vials 174 and 176 are held ina level position by their supporting beam 166. If the lefthand side ofthe beam 166 drop the electrodes 178 and 180 will be conducting and theelectrodes 182 and 184 are non-conducting. If the right-hand side of thebeam 166 drops the pair of electrodes 182 and 184 will be conducting andthe electrodes 178 and 180 will be nonconducting. Upon restoration ofthe beam 166 to a level position the illustrated condition will bere-established, in which neither pair of electrodes will be conducting.

An electronic control circuit responsive to conditions of the vials 174and 176 serves to operate the clutches 156 and 158 as is necessary tomaintain the beam 166, and consequently the axis of pressure drum 4,exactly horizontal. Each of the vials 174 and 176 has its own,

6 independently operating control circuit. These circuits are identicaland, therefore, for brevity of description only the circuit associatedwith the vial 176 will be described in detail.

Three transformer secondaries, T1, T2 and T3, are supplied from a singleprimary which is connected to an A.C. supply. T1 supplies 24 volts A.C.across both the vial 176 and a high resistance R1 (680,000 ohm). T2supplies a lower A.C. voltage (6.3 volts) which is rectified by arectifier RTl. The voltage from T1 is rectified by a rectifier RTZ butcharges a capacitor C2, with the voltage across C2 also appearing acrossa resistor R3 (1,000 ohm). Transformer secondary T1 is connected inphase with its primary as indicated, and supplies the triggering voltagefor a thyratron T in phase with the plate voltage thereof. The controlgrid circuit for thyratron T comprises the resistor R1, resistor R2, apotentiometer P, and normally closed relay contacts CRlc, and anyvoltage across these elements appears as a biasing voltage for thyratronT.

When vial 176 is level, thyratron T will be non-conducting due to anegative bias across the resistor R2, which has its negative sideconnected to the control grid through resistor R1 and its positive sideconnected to the cathode through potentiometer P, relay contacts CRlcand a cathode resistor R4. This negative bias is sufficient to keep thethyratron from conducting. The bubble B in vial 176 will then be underthe electrode 180, and offectively all of the 24 volts A.C. fromtransformer secondary T1 will be dropped across the vial. If the sandingmachine then approaches an area causing the lefthand side to droprelative to the right-hand side, the bubble B will shift (uphill) towardthe right and beyond the electrode 180, thereby removing the insulatingair gap from below this electrode. The resistance of the vial, i.e.across liquid L and electrodes 178 and 180, will then drop to the pointwhere approximately 12 volts A.C. will override the negative bias atresistor R1. (On the other hand, if the left-hand side rises in relationto the right, the bubble B will remain below electrode and there will beno change of negative bias.) Inasmuch as the overriding A.C. voltage isin phase with the thyratron plate voltage, this tube will conduct onevery positive half cycle.

On each pulsation of thyratron T, a relay coil CR1 will be energized anda capacitor C3 in parallel will be charged. This relay has normally opencontacts CRla and CRld and normally closed contacts CR1!) and CRlc. Asevident from the wiring diagram, energizaton of coil CR1 due to pulsingof thyratron tube T will effect closing of the contacts CRla which arein series with the electromagnetic clutch 158. As previously described,upon energization of this clutch it will be engaged to drive the leadscrew 128 in a direction to raise the left-hand end and lower theright-hand end of drum 4.

When relay contacts CRlc open the bypass in the grid circuit for the 24Volt DC. power supply is removed, and as relay contacts CRld close thispower supply is applied across capacitor C4 through a portion ofpotentiometer P. This voltage is applied in the thyratron grid circuitcomprising resistors R1 and R2, and capacitor C4, and accumulates untilthe grid is sufficiently negative that the A.C. signal from resistor R1cannot cause the tube T to conduct. This state will effectde-energization of relay coil CR1. The contacts CRla will then open andclutch 158 will disengage. Also, capacitor C4 will now discharge througha portion of potentiometer P and through relay contacts CRld, resultingthereafter in restoration of the DC. negative signal at resistor R1.Assuming that the short duration of operation of clutch 158, in responseto the one pulse of thyratron tube T, was not sufficient to bring thecut right vial 176 back to level condition, the bubble B will not haveretreated suificiently to insulate electrode 180 from liquid L. Ac-

cordingly, the AC. voltage supplied by transformer secondary T1 will beapplied across vial 176 to override the negative bias at resistor R1,and another pulse of thyratron T will again result to re-energize relaycoil CR1. This pulsing of the thyratron tube T will continue untilsuccessive engagements of the clutch 158 have caused driving movementsof lead screw 128 sufficient to return the pressure drum 4 (and also thebeam 166 on which the vials 174 and 176 are mounted) back to an exactlyhorizontal position.

As evident from the foregoing description, according to the inventionthere is involved a pulse type of operation as distinguished fromcontinuous operation of a mechanism throughout a duration sufiicient toeffect proper levelling. Certain advantages of this feature will betreated hereafter. In practice, the pulsations occur in discrete on-offcycles of operation of approximately one second each during operation toeffect a levelling correction, although it will be understood that theinvention is not limited in scope to this specific timing.

The circuit (as shown at the lower right-hand corner of the wiringdiagram) associated with the cut-left vial 174 is identical with thatjust described in connection with the cut-right vial 176. In the circuitfor the cutright vial 176 the counterpart of relay coil CR1 is relaycoil CR2, which is energized by the pulsating conduction of a thyratrontube when the vial 174 is caused to conduct an overriding AC. voltagedue to dropping of the right-hand side of beam 166 relative to theleft-hand side. When this occurs the contacts CR2a are closed by theenergization of relay coil CR2 and, the contacts CRlb then being closedas normal, the electromagnetic clutch 156 is engaged. Again, as in thecase of energization of electromagnetic clutch 158, the clutch 156 willundergo intermittent engagements in response to the repettive on-or'fpulsations of the thyratron tube which controls relay coil CR2. It issignificant to note, for reasons discussed hereafter, that each suchpulse results in a very minute levelling adjustment of the pressure drum4.

Certain interrelated aspects of operation of the relay contacts CRla,CRlb, CRZa and CR2!) may be noted. In series with contacts CR1b and CRZbis a lamp 186, the function of which is to signal that the machine issanding on the level and that neither of the vials 174 and 176 issensing a deviation of drum 4 from the horizontal. Accordingly, becauseof this series connection, when neither of the circuits associated withthe respective vials 174 and 176 is operating, both sets of contactsCRlb and CR2b will be in their normally closed positions to causeillumination of lamp 186. This illumination will be interrupted uponoperation of either of these circuits to eifect opening of one of thesets of contacts CR1b and CR2b. A lamp 188 is provided to indicate thatthe left-hand side of the pressure drum 4 has fallen relative to theright-hand side and that in response to the sensing of this condition bythe vial 176 the levelling system is in operation to correct thecondition. This illumina tion (a flickering in time with the clutchengagements) occurs, of course, because the lamp 188 is connected inparallel with the cut-right clutch 158. A lamp 190 is provided toindicate that the right-hand side of the pressure drum 4 has fallenrelative to the left-hand side and that in response to the sensing ofthis condition by the vial 174 the levelling system is in operation tocorrect the condition. Lamp 190, being connected in parallel withelectromagnetic clutch 156, will flicker in time with its repetitiveengagements.

Concluding the above description, the electronic levelling system hasproved itself to be highly sensitive and accurate. In addition, thepulse type of operation is highly advantageous in connection with otheraspects. It has been found that in a sanding machine wherein accuracy inlevelling is very important, as certain devices are brought into play toeffect levelling adjustments of a pressure drum or the like relative toa main framework,

variable stresses set up in the machine can raise problems. For example,as the means for effecting a levelling correction begin to operate inresponse to the level sensing means, the reaction to the involvedrelative movements produces perhaps infinitesimal but neverthlesssignificant variable stresses. The sensing means sense the absoluteposition of the cutting parts when in the stressed rather than thenormal or unstressed, condition. Therefore, when the sensing meanssignal that the cutting parts have been restored to an exactlyhorizontal position, the means for effecting this levelling correctionwill cease to operate. When this occurs a relaxation of the additionalstresses takes place which can produce an overshoot beyond the intendedresting place at the exactly horizontal position. However, by virtue ofthe present pulse system this does not occur because there is noopportunity for any sizeable accumulation of distortion in the frame.Such accumulations tending to produce overshoot do not occur with theuse of discrete pulses which operate either of the electromagneticclutches intermittently.

The levelling system in accordance with the invention has been describedin its application to a sanding machine of the type used in thefinishing of bowling lanes, where the problem of automatic and accuratelevelling is of particular concern. However, within the scope of theinvention this system is applicable to other arts in which meansresponsive to inclination are desired for the production of indicatingsignals or the operation of external components.

Various other departures from the specifically disclosed embodiment ofthe invention can be effected without departing from the scope thereofas defined by the following claims.

What is claimed is:

1. In a sanding machine, a travelling main machine frame, an assemblyincluding a travelling abrasive member carried by said frame, means forsensing the position of said assembly in relation to the horizontal,means for shifting the angular relation of said assembly relative tosaid main frame and the horizontal, and means responsive to said sensingmeans upon a deviation of said assembly in either direction from thehorizontal for operating the last-mentioned means, in the direction tocorrect said deviation, in discrete, closely spaced and gradual stepscontinuing until said horizontal position is restored.

2. In a sanding machine, a travelling abrasive member, means operablefor corrective adjustments of the cutting angle of said member relativeto a horizontal reference, control means for the last-mentioned meansincluding first electrically driven means for effecting correctiveadjustments in one direction and second electrically driven means foreffecting corrective adjustments in an opposite direction, bubble-typelevel sensing means containing elecrically conductive liquid andelectrodes each being positioned in the path of shifting of a bubble inone of two opposite directions whereby titling produces changes inconductivity of the electrical circuit including conductive liquid andan electrode, independent electrical control circuits, each of saidcontrol circuits being coupled between and associated with one of saidelectrically driven means and one of said electrodes, and beingresponsive to an aforesaid change in conductivity thereof to supplypower to its associated electrically driven means until said referenceposition is restored, each of said circuits comprising a thyratron,means normally applying to the thyratron a bias voltage to preventfiring of the tube and means responsive to firing of the thyratron tubeto transmit driving current to said associated electrical means, thesaid level coupled to the control circuit being adapted to remove saidbias voltage upon a change in conductivity due to deviation of the levelfrom the horizontal reference.

3. The invention according to claim 2, wherein said means operable forcorrective adjustments comprise means constantly driven in one directionand reversible means for effecting the actual adjustment in eitherdirection, and wherein said electrically driven means comprise clutchesconnected between said constantly driven 10 in the path of shifting ofthe bubble, whereby tilting of each level produces changes inconductivity of the electrical circuit including its electrode andconductive liquid, independent electrical circuits, each of saidcircuits being means and said reversible means, the engagements of 5coupled between and associated with one of said elecsaid clutcheseffecting opposite movements of said reversible means.

4. In a sanding machine, a travelling abrasive member, means operablefor corrective adjustments of the cutting angle of said member relativeto a horizontal reference, control means for the last-mentioned meansincluding first electrically driven means for effecting correctiveadjustments in one direction and second electrically driven means foreffecting corrective adjustments in an opposite direction, bubble-typelevel sensing means containing electrically conductive liquid andelectrodes each being positioned in the path of shifting of a bubble inone of two opposite directions, whereby tilting produces changes inconductivity of the electrical circuit including conductive liquid andan electrode, independent electrical control circuits, each of saidcontrol circuits being coupled between and associated with one of saidelectrically driven means and one of said levels, and being responsiveto an aforesaid change in conductivity thereof to supply power to itsassociated electrically driven means until said reference position isrestored, each said circuit comprising a power circuit producing closelyspaced discrete current pulses to operate its associated electricallydriven means in closely spaced gradual steps.

5. In a sanding machine, a travelling abrasive member, means operablefor corrective adjustments of the cutting angle of said member relativeto a horizontal reference, control means for the last-mentioned meansincluding first electrically driven means for effecting correctiveadjustments in one direction and second electrically driven means foreffecting corrective adjustments in an opposite direction, a pair ofbubble-type levels each containing an electrically conductive liquid andan electrode positioned trically drive means and one of said levels, andbeing responsive to an aforesaid change in conductivity thereof tosupply closely spaced discrete current pulses to its associatedelectrically driven means to operate the same intermittently in gradualsteps until said reference position is restored.

6. The invention according to claim 5, wherein said sanding machinecomprises a travelling main frame and a member providing a backing forsaid abrasive member and determining the angle of cutting thereofrelative to said main frame, said member being adjustably mounted insaid main frame and subject to the operation of said means for effectingcorrective adjustments.

References Cited by the Examiner UNITED STATES PATENTS 977,523 12/10Gustafson 200152 1,375,278 4/21 Clayton 200-152 X 1,465,500 8/23 Vance200-152 X 1,676,791 7/28 Mailey 200152 2,338,811 1/44 Hasbrook.

2,446,180 8/48 Haskins 200152.7 X 2,477,927 8/49 Hanson 200-1522,502,217 3/50 Guibor 200152 X 2,688,217 9/54 Winkler et al. 51-1743,020,506 2/62 Remington et a1. 200-152 X 3,030,477 4/62 Hensley 200152LESTER M. SWINGLE, Primary Examiner. FRANK H. BRONAUGH, J. SPENCER OVER-HOLSER, Examiners.

1. IN A SANDING MACHINE, A TRAVELLING MAIN MACHINE FRAME, AN ASSEMBLYINCLUDING A TRAVELLING ABRASIVE MEMBER CARRIED BY SAID FRAME, MEANS FORSENSING THE POSITION OF SAID ASSEMBLY IN RELATION TO THE HORIZONTAL,MEANS FOR SHIFTING THE NGULAR RELATION OF SAID ASSEMBLY RELATIVE TO SAIDMAIN FRAME AND THE HORIZONTAL, AND MEANS RESPONSIVE TO SAID SENSINGMEANS UPON A DEVIATION OF SAID ASSEMBLY IN EITHER DIRECTION FROM THEHORIZONTAL FOR OPERATING THE LAST-MENTIONED MEANS, IN THE DIRECTION TOCORRECT SAID DEVIATION, IN DISCRETE, CLOSELY SPACED AND GRADUAL STEPSCONTINUING UNITL SAID HORIZONTAL POSITION IS RESTORED.